Perspectives Without Physical Grounds: A Critique of Embodiment and Local Unitarity in Relational Quantum Mechanics

Di Biagio and Rovelli’s recent reformulation of Relational Quantum Mechanics (RQM) introduces relative facts grounded in commutative subalgebras and relative Shannoninformation, claiming to provide a physically embodied, naturalistic, and locally coherent account of quantum phenomena. We identify a structural inconsistency in this programme by analysing a scenario in which observer A remains entirely unperturbed while a remote observer B measures a complementary observable of a shared entangled system S. We prove rigorously that (i) A’s local reduced density matrix is invariant under this remote action, (ii) the relative information I(Z|A) nonetheless drops from maximal to minimal, and (iii) no local unitary acting on H(A) can generate this change, since local unitaries preserve mutual information. These three results jointly entail that the evolution of A’s perspective is not generated by any local physical process, directly contradicting the paper’s explicit commitment to physical embodiment and local autonomy of perspectives. We argue this is not a technical defect but a foundational tension: RQM cannot simultaneously maintain that perspectives are locally embodied in physical correlations, that they evolve unitarily when no interaction occurs, and that remote complementary measurements alter local relative facts. Without additional structure specifying the dynamics ofperspectives under non-local entanglement changes, RQM’s naturalistic programmeis internally inconsistent at this level. We further argue that no-signaling doesnot rescue the ontological claim, and compare the problem’s severity to analogous nonlocality in other interpretations.